This invention relates to grate construction for a fixed bed coal gasifier. In the fixed bed coal gasification process, coal enters the gasifier at the top and lands on top of the charge of coal already in the gasifier where it receives heat from the upwardly-moving product gas. As the combustion process near the bottom of the gasifier (above the grate) consumes some of the coal and the rest of the coal is gasified, coal at the top of the charge gradually moves down, passing through a series of treatment stages: initial heating; devolatilization and coking; gasification, and carbon oxidation. A large number of chemical reactions occur and a minimum temperature of about 1700.degree.-1900.degree. F. is required. Usually the gasification is conducted under superatmospheric pressure. When air and steam are introduced to the combustion zone, the end product is producer gas; when oxygen and steam are employed, the end product is synthesis gas.
It is well recognized in the coal gasification art that problems arising in the operation of coal gasifiers are often connected with poor control over the solids flow distribution down the gasifier shaft, which condition is determined by the grate construction. The state of the art grate constructions provide only limited control over the discharge of solids from the grate at preselected radial positions and no grate construction known provides control over the discharge of solids from preselected sectorial positions.
U.S. Pat. No. 2,440,940--Galusha described an eccentrically stepped grate construction, the rotation of which results in ash (or other solids) gradually being dropped through the grating around the edge and between the step grates. A similar grate is also disclosed in U.S. Pat. No. 3,454,382--Hamilton. The Hamilton patent also describes the use of temperature-sensing means mounted on the grate.
The Junkers patent (U.S. Pat. No. 2,808,321) is directed to a rotating grate construction provided at its underside with sector-shaped compartments adapted to receive independently controllable flows of a gasification medium in order to maintain a uniform reaction rate in the charge over the cross-section of the producer shaft.
It would, therefore, be of particular advantage, and is an object of this invention, to provide grate construction which makes possible improved control over solids discharge at preselected radial positions whereby the by-pass of unburned carbon through the grate is substantially reduced.